The first step to relax the rather severe restrictions of the bare Zwanzig model, is to deform somehow the reservoir equidistant
spectrum. To illustrate how it works in the following subsection REF  we study only two special types of the spectrum deformations,
and then, armed by this knowledge in subsection REF  we investigate more general spectral deformations, enabling to describe
almost arbitrary reservoir spectra. It is worth to note that if the order of the reservoir levels does not change upon deformations
\((\varepsilon _{n+1}^0 - \varepsilon _n^0 > 0)\)  (i.e., for any \(n\) , there are no level permutations), the spectrum remains regular.
If it is not the case, mixing of the levels occurs. One should distinguish this spectral mixing, producing chaotic behavior in quantum mechanic
systems, and also known dynamic mixing phenomena leading to chaotic classical dynamics [1]} - [2]}.
